German submarine U-775
Summary of raiding history

Date
Name
Nationality
Tonnage
Fate
6 December 1944
HMS Bullen
Royal Navy
1,300
Sunk
28 February 1945
Soreldoc
United States
1,926
Sunk
6 March 1945
Empire Geraint
United Kingdom
6,991
Damaged

Question: How effective were the U-775 submarines during the war? 
Answer: German submarine U-775 sank one merchant ship, for a total of 1,926 gross register tons (GRT), one warship sunk (1,300 tons) and one merchant ship damaged (6,991 GRT).